1. Field of the Invention
The present invention relates to a capacitive card module which has a plurality of capacitive switch elements arranged side-by-side on an insulating carrier layer and which are connected to one another and respectively composed of conductor surfaces allocated to one another and fashioned into capacitor surfaces.
2. Description of the Prior Art
Capacitive card modules are being utilized to an increasing degree in newly-designed keyboards of professional data processing technology in order to improve the reliability of the keyboards and in order to simultaneously reduce the manufacturing cost.
European patent application Serial No. 88 11 3995.0, for example, discloses a keyboard mechanism wherein a key plate, together with a frame element and a seating part, is fashioned as a one-piece formed part and whereby a capacitive card module is directly secured to the underside of the key plate.
Compared to traditional keyboards whose keys act on switch elements that, via a respective galvanic contact, connect a row line and a column line to one another in a key matrix, the row and column lines in keyboards having capacitive switch elements are not galvanically connected to one another.
For this reason, decoupling elements such as, for example, respective diodes connected in series with the galvanic contacts are likewise not required.
A capacitive switch element can be realized in a simple manner with two conductor surfaces fashioned on a conductor card to form capacitor surfaces these conductor surfaces being connected to respective row lines or, respectively, column line of the key matrix. The actuation of a key merely produces a variation of the capacitance of an appertaining capacitive switch element. This can either occur on the basis of the basis of the approach of a dielectric element or, on the other hand, by reducing the distance between the capacitor surfaces can likewise occur by enlarging one of the capacitor surfaces.
A change in capacitance between row lines and column lines can be recognized with the assistance of an evaluation electronic circuit connected to all row and column lines and an actuated key can be identified with reference to the participating lines.
Function outages due to oxidized or deformed galvanic contacts, as well as due to malfunctioning decoupling element, can therefore not occur in keyboards having capacitive switch elements.
Since capacitive switch elements are composed only of individual conductor surfaces and the conductor surfaces are usually connected to one another via row and column lines, it was obvious for the purpose of reducing costs to provide a common printed circuitboard for all capacitive switch elements of a keyboard, no other electronic component being located on the printed circuitboard. As a rule, such a printed circuitboard is fashioned as a flexible PC foil and is then referred to as a capacitive switch foil.
Involved evaluation electronics is required for such capacitive switch foils obtainable in a great variety of embodiments, since only a very slight change in capacitance occurs as a rule given actuation of a capacitive switch element. Moreover, the change in capacitance to be recognized given actuation of a key is further reduced by parasitic coupling capacitances that can form due to interconnects extending in parallel to one another due to capacitive switching elements lying in tight proximity to one another.
It is therefore an object of the present invention to improve a capacitive card module of the type set forth above such that the influence of parasitic coupling capacitances is reduced.
The above object is achieved, according to the present invention in a capacitive card module having a plurality of capacitive switch elements arranged side-by-side on an insulated carrier member and connected to one another, the capacitive switch elements being respectively composed of conductor surfaces allocated to one another and fashioned into capacitor surfaces, and is characterized in that the insulating carrier layer comprises an electrically conductive first shielding surface fashioned as an interconnect grid at its side facing away from the capacitive switch elements and is provided with a terminal for voltage potential, in that the first shielding surface comprises a second shielding surface in mesh regions which are in electrical contact with the first shielding surface, in that the first shielding surface is composed of a conductor material that is electrically more conductive than that of the second shielding surfaces. According to another feature of the invention, the second shielding surfaces form a closed area and cover the first shielding surface at at least one side.
According to another feature of the invention, the conductor material of the first shielding surface is composed of silver and that of the second shielding surface is composed of graphite.
The expense for the evaluation electronics can be kept low with a card module constructed in accordance with the present invention. The reason for this become clear only given the assumption that the interconnect surfaces of the first shielding surface are negligible overall compared to the surface covered by the second shielding surfaces. It is then assured, in particular, that the two arbitrary lines (row-column lines) of the capacitive card module between which a parasitic coupling capacitance may exist under certain circumstances no longer have any coupling capacitances to the first shielding surface, but do have to the second shielding surfaces.
In principle, coupling capacitances to shielding surfaces are desired so that the electrical noise charges can be coupled by parasitic coupling capacitances from a disturbing line onto a disturbed line can flow off to a voltage potential, usually to ground potential. This is possible in the capacitive card module constructed in accordance with the present invention because the second shielding surfaces are electrically connected to the first shielding surface and the latter is connected to a voltage potential (ground potential). Since, then, both the disturbing as well as the disturbed lines are each connected to the voltage potential via coupling capacitances, these two coupling capacitances are fundamentally connected in series and are therefore connected in parallel to the parasitic coupling capacitance. What is disadvantageous is that the parasitic coupling capacitance is thereby further increased and electrical noise charges can increasingly flow onto the disturbed line.
In the ideal case, electrical noise charges should flow off via the coupling capacitance to the shielding surface and should then flow optimally unimpeded to a voltage potential (ground potential). Electrical noise charges, however, should be impeded from flowing via the coupling capacitances and a shielding surface from one line to another line, for example from a row line onto a column line.
This is achieved in the card module constructed in accordance with the present invention in that the second shielding surfaces have a lower electrical conductivity when compared to the first shielding surface. Electrical noise charges that proceed via a coupling capacitance onto the second shielding surfaces must overcome a certain resistance (for example 10 ohms given graphite) dependent on the electrical conductivity of the second shielding surface until they arrive onto an interconnect of the first shielding surface in which they can then flow off to the voltage potential without impediment (extremely good conductivity, for example 1 ohm given silver). In order to return via a coupling capacitance onto a line, the electrical noise charges would then again have to overcome the resistance of the second shielding surfaces, for which reason the plurality of noise charges proceeding via the shielding surfaces onto a line can be kept low with a card module constructed in accordance with the present invention compared to what is possible given card modules that comprise only one shielding surface composed of a single continuous area.
The conductor materials of silver and graphite can be galvanically combined with one another without problems and also well-suited for being applied in a silk screening process. In order to protect the unusually thin interconnects of the interconnect grid, it is advantageous to fashion the second conductor surfaces as a closed surface and to cover the interconnect grid as a whole. Moreover, the expense for the application of the second shielding surfaces in a silk screening process is thus reduced.